With the rising popularity of digital cameras and mobile phones equipped with cameras in recent years, private photographs have come to be handled in the form of image data more frequently. Also, the image data is often imported from a digital camera or a mobile phone equipped with a camera either for display on a television or other display device or to print on a printer or other image forming device. Accordingly, cable-free, wireless communications technology is now in popular use for transmission of relatively large data, such as video and audio, between information devices over a PAN (Personal Area Network) which provides a coverage of a few meters.
The conventional communications standards used in the communications field mentioned above are built on the OSI (Open System Interconnection) model shown in FIG. 12 to ensure data communications between different types of devices. An example is the IrDA® (Infrared Data Association) standard. The conventional communications standards are basically for bidirectional communications in which an operation is repeated to check that both devices are ready to communicate with each other by searching for a device with which to communicate, identifying it, and negotiating with it.
Different specifications are established for each layer in the communications standards based on the OSI model. Specifically, the OSI model is designed to include: a data link layer managing a protocol for communications with another device; a network layer managing a data link by means of services given by the data link layer; a transport layer managing flows of data frames by means of frame numbers to detect missing frames; and a session layer. The layers of the communications device exchange information, including a connect request, a connect command, and a connect acknowledge, to establish/discontinue a connection as well as to transfer data.
Meanwhile, in the use described above, the user often directs by himself the light emission section of a transmitting-end device at the light reception section of a receiving-end device to transmit data. Therefore, the bidirectional process of searching for a device with which to communicate and ascertaining each other is not always necessarily.
Japanese Unexamined Patent Publication No. 2005-347836 (Tokukai 2005-347836; published Dec. 15, 2005) discloses a communications system in which management information based on which data to be transmitted is managed is transmitted, communications capability information indicative of the communications capability of a device which has responded to the transmission of the management information is received from that device, and data to be transmitted is transmitted in packets according to the received communications capability information, so as to omit the process of searching a device which is capable of receiving data in infrared communications.
There is also a communications standard called IrSimplee which realizes high speed data communications when the process of searching for a device which is capable of reception is not absolutely needed in infrared communications. See “Standardization of High Speed Infrared Communications Protocol IrSimple,” by NAOE Hitoshi and six other authors, Sharp Technical Report, Issue 95, February 2007, pp. 63-68.
Referring to FIG. 13, in conventional infrared communications, a connect frame (communications information notification frame) in SIR (Serial Infrared) communications mode is transmitted at low speed (about 9,600 bps) prior to the transmission of data frames in FIR (Fast Infrared) communications mode at high speed (than 4 Mbps or faster). Generally, a data frame is transferred more than once when image data or like large data is transferred. When the transfer of data frames is finished, a DISC (Disconnect) frame, which is a disconnect command in FIR communications mode, is transferred to discontinue the communications connection.
A “frame” in this context refers to a frame used generally in the communications field. It is a piece of information having its start and end being defined.
A “connect frame” is a set of information needed to enable communications between a communications device on a transmitting-end and a communications device on a receiving-end.
A “data frame” is a set of information containing image data or a similar content which is to be transferred between a communications device on a transmitting-end and a communications device on a receiving-end.
A “frame format” defines a frame structure, specifying which bit represents what in the sequence of bits in a frame.
An SNRM frame (connect frame for IrDA, IrSimple, etc.), taken as an example, has a frame format shown in FIG. 14. The ADR segment gives a broadcast address. The SNRM segment gives data representing a frame type, indicating that this frame is an SNRM frame.
The Source Address segment gives data indicative of the address of a primary station (communications device on a transmitting-end). The Destination Address segment gives data indicative of the address of a secondary station (communications device on a receiving-end). The Connection Address segment gives data indicative of an address used in communications with a station on another end. The connection parameter segment contains data indicative of transmission conditions for the succeeding data frame.
In contrast, a UI frame (IrSimple data frame) has a frame format shown in FIG. 15. The ADR segment contains a Connection Address given by an SNRM frame which is an address used in communications with a station on another end.
The UI segment gives data representing a frame type, indicating that this frame is a UI frame. The transmission data segment a set of data containing image data or a similar content which is to be transferred. The frame format does not vary depending on communications speed and modulation schemes.
Conventional infrared communications are prone to infrared noise emitted by, for example, an inverter-type fluorescence lamp, a liquid crystal television containing a cold cathode tube, and a plasma television. The receiving-end device may not be able to receive a low-speed connect frame correctly, and as a result, fail to properly receive data frames which follow the connect frame.
In the case of a liquid crystal television with a cold cathode tube or a like communications device emitting infrared noise, the emitted infrared noise is reflected by objects located nearby and obstructs infrared communications. The infrared noise reflected by the body of the user is especially disruptive in infrared communications because the user operates the transmitting-end terminal while holding it in his hand as close as about 20 cm to 1 meter to the infrared reception section of the receiving-end communications device. It is also known that infrared noise increases if the temperature of the cold cathode tube is as low as 0° C. to 10° C. or even lower. It is also recognized that in some cases it takes about 10 minutes for the cold cathode tube to warm up enough to reduce effect of infrared noise after the television is powered on in such a low temperature environment.
The origin of the trouble caused by infrared noise in data reception in conventional infrared communications can be traced back to the fact that a UI frame (data frame) is transmitted in FIR communications mode after an SNRM frame (connect frame) is transmitted in SIR communications mode.
Specifically, data bits 0 and 1 are represented by the presence/absence of a pulse in a predetermined period in the RZI (Return to Zero Inversion) modulation employed in SIR communications mode. For example, the IrSS (“IrSimpleShot®”; a one-way communications standard under the IrSimple), in SIR communications mode in which a connect frame is transmitted at a communications speed of about 9,600 bps, specifies an insertion of a 1.41 μs to 22.13 μs pulse in an about 104 μs period to represent a data bit 0 and no insertion of a pulse in the about 104 μs period to represent a data bit 1.
Hence, the connect frame is masked by noise signals if, for example, an inverter-type fluorescence lamp exists near the communications device and producing lot of infrared noise pulses with a duration of about a few microseconds to a few tens of microseconds. The noise pulses are erroneously recognized as data bit pulses, the connect frame cannot be correctly received. As a result, the communications device on the receiving-end cannot switch to data frame reception in FIR communications mode, failing to receive data frames transmitted in FIR communications mode.
The problems could be addressed by sending a connect frame in a noise-resistant FIR communications mode or introducing an error correction code, e.g. Reed-Solomon error recovery code, to improve noise resistance without altering modulation schemes. it is, however, not easy to alter the signal formats of the connect frame, which is transmitted before any other frame, due to the need to provide compatibility with conventional standards.
The description above has dealt with issues in the transmission/reception of a connect frame and data frames by infrared. The same issues are found with a communications device which, prior to the transmission of data frames, transmits a connect frame containing data frame transmission conditions in a different signal format from the one in which the data frames are transmitted.